The drawback of this arrangement is that the position at which the ribs should be fitted is dependent on the pressure differences between the two face-plate ports, and since these pressure differences are not fixed, measures have to be taken to ensure correct operation in the event of differing pressure differences. These measures generally comprise the fitting of leakage grooves or a brief short circuit between the rotor ports by narrowing the rib, so that a chamber is simultaneously in communication with both rotor ports. This reduces the delivery while still not offering a good solution for all situations.